1. Field of the Invention
The invention concerns a power stage for driving an electric machine.
2. Description of Prior Art
A power stage for driving a brushless electric motor is known in the field. It comprises several semiconductor switches designed as transistor chips, of which two each are joined by bond wires and conductor tracks to form a half bridge. The semiconductor switches are arranged on a printed circuit board comprising an electrically insulating substrate, onto the surface of which that is facing the semiconductor switches conductor tracks of copper are deposited. The semiconductor switches have on their front and rear faces electric contact areas which, on the rear face of the semiconductor switch facing the circuit board, are connected to the conductor track. To each contact area on the front face of the semiconductor switches bond wires are attached by ultrasonic welding. The outer edges of the circuit board are surrounded by a frame which comprises the electric input and output terminals for connection to a motor winding and/or a power supply. These input and output terminals are connected to the conductor tracks of the circuit board by bond wires. The bond wires have a diameter of approx. 0.3˜0.5 mm and therefore a relatively large ohmic resistance which can reach more than 1 mΩ. Thus, the electric resistance of the bond wires lies in the approximate range of a low resistance semiconductor switch, i.e. the bond wires contribute already up to 50% to the total resistance of the power stage. This is unfavorable, particularly for driving of electric motors which require a relatively high motor current, from which follows the requirement that the motor circuit and thus the power stage is to be designed for as low resistance as possible.
Another disadvantage of the power stage is the need of the bond wires for large space on the circuit board, since they, besides the semiconductor switches, require a contact surface . The circuit board surface of the power stage amounts to a multiple of the chip surface of the semiconductor switch. Therefore, a relatively large amount of conductor and insulation material is required for the production of power stages, which increases costs. In addition to this, the costs for the bond wires has to be considered. Finally, ultrasonic welding can not always guarantee a secure contact of the bond wires with the conductor tracks, respectively the semiconductor switches.
Therefore, the objective of this invention is to create a power stage of the above indicated type which provides low impedance and high operational reliability. Furthermore, the output performer should have compact dimensions and its production should be cost effective.